bitpicker
Какой / каков ask for one piece of information out of a number, such as 'what suggestions do you have?' - note that in English you can replace the 'what' in the second sentence with 'which', but not the one in the first. So I would say you can use forms of каков when you can use 'which' instead of 'what' in English.
No, you can't.

каков is very rare and limited mostly to an archaic or bookish style, it is used to ask what smt is like. You might go through life and never have to use this word or bump into it.

'Which' and 'what' are both translated as 'какой'. The difference is only figured out from context.